The present invention is an improvement in a reflex camera focusing screen having vernier focusing. Vernier focusing, also referred to as split image ranging, relates to an optical arrangement wherein part of the image seen on the screen is misaligned, or split, with respect to itself, except when the image is focused exactly on the screen. The construction of the reflex camera is such that sharp focus on its focusing screen corresponds to sharp focus on its film plane.
The usefulness of a vernier focusing means lies in the ability of a user to more readily perceive the alignment of the image's parts than the sharpness of the image.
U.S. Pat. Nos. 2,881,686; 2,914,997; 2,980,001; and 3,498,198 illustrate a variety of focusing screens that are typical of the prior art. They all show vernier focusing means located in a small circle at the center of the focusing screen. One of them suggests a Fresnel mirror focusing screen with the vernier in the center comprising different Fresnel mirror elements.
The common fault of the prior art devices that this invention redresses is the effect of the vernier circle's central location on photographic composition. The central location induces a photographer to unconsciously rely on the vernier as a bull's-eye. This has a subtle but detrimental effect on the aesthetic composition of his photograph because he tends to merely center his subject with the vernier circle rather than affirmatively evaluating the composition on its aesthetic merits.
Another recognized fault of the prior art is the adverse effect the central location has on image acuity at the edge of the field. Camera objective lenses do not usually have flat fields. Their fields curve. The best focus of the curved field (focal surface) on a flat image surface is based on a compromise between zonal aberrations at the center (axis) of the field and the curved field off-axis. However, if the center of the image is sharply focused at the film plane, by using a split image means, the edge of an inwardly curving image will be focused well in front of the focal plane and be unsharp at the focal plane.
FIG. 5 illustrates the prior art, i,e., a flat focusing screen with a centrally located vernier means (in this instance crossed prisms or wedges to provide a split image feature). The off-axis portions of the focal surface curve markedly. Consequently, the image on the focusing screen, away from the center, is blurred.
An objective lens, by design, generally achieves improved overall results (sharpness) when the region of best focus is part way between the center of the field and the edge. Thus, the objective lens' curved image surface will intersect the film plane between the center and the edge of the field so the edge focuses slightly in front of the plane and the center focuses slightly behind the plane. This minimizes the possible loss in sharpness between the center and the edge of the field due to image curvature. Also, the region of best focus then encompasses a proportionally larger area. Locating the vernier means substantially off-center places it where it will yield best focus for the proportionally larger area and minimizes the possibility of an image sharply focused at the center but noticeably unsharp at its edges.
U.S. Pat. No. 3,699,867 should be noted. It shows a viewfinder for a popularly priced camera with a red square above the center of the field. The red square is a stadimeter rangefinder element. A photographer uses it by adjusting his distance from his subject so the subject's face fills the red square. He then knows he is separated by a predetermined distance from his subject. A portrait usually has the person's face above center; therefore, this viewfinder has the red square above center as a convenience for the photographer.